The invention relates to an internal combustion engine comprising a cylinder with a spark plug, an electric DC voltage supply and an ignition circuit.
Internal combustion engines provided with spark plugs for igniting a gas mixture in a cylinder are generally known. Such engines are provided with an ignition coil, which is essentially a transformer with a switch between a primary winding of the transformer and an electric supply such as an accumulator and a coupling of the secondary winding to the spark plug. The ignition coil provides for a high voltage across the electrodes of the spark plug, which leads to a spark-over so that the gas mixture is ignited.
U.S. Pat. No. 5,456,241 discloses an ignition circuit for an internal combustion engine. This ignition circuit includes a capacitor which forms a resonant circuit with a series connection of the primary winding and a supplementary coil. To generate high voltages, first the capacitor is charged, while a switch keeps the current path through the primary winding and the supplementary coil interrupted. Next, the switch is rendered conductive. This leads to a dampened oscillation with voltage peaks which, via the transformer, lead to the desired high voltages across the electrodes of the spark plug. Upon the first voltage peak, an electric breakdown through the gas mixture occurs, so that a breakdown path through the gas mixture between the electrodes becomes conductive. Next, the rest of the oscillations lead to an oscillating current through the breakdown path. Thus, the gas mixture is heated to a temperature at which combustion occurs. An important parameter of such an ignition is the energy thereby transferred to the spark plug. Further, erosion of the spark plugs resulting from the discharge impact is an important parameter. U.S. Pat. No. 5,456,241 describes how the switch is made conductive and non-conductive several times for the same combustion. Thus, the capacitor can be charged several times and more energy is transferred to the spark plug than through switching the switch a single time.
U.S. Pat. No. 5,456,241 further describes how the amount of energy that can be transferred from the supply to the gas mixture decreases according as the speed of the motor increases. While this effect is counteracted by increasing the frequency with which the switch is switched on and off at higher speeds, the amount of energy nonetheless decreases at higher speeds. This is because at high speeds the time between consecutive combustions of the gas mixture more and more approximates the charging time needed to charge the resonant circuit. This charging time is fairly great, because a fairly large self-induction of the primary winding is needed for the storage of sufficient energy for igniting the gas mixture.
U.S. Pat. No. 4,677,960 describes a capacitive discharge ignition circuit using a high efficiency voltage doubling ignition coil. The ignition is used with a high pulse rate, multiple pulse ignition box providing rapid pulsed plasma ignition sites. The circuit may include an energy storage capacitor but the coil will also operate with standard or electronic ignition excepting that full advantage cannot be taken of the high current/voltage capabilities of the coil since these ignitions cannot store high energy rapidly. Because the coil comprises a core made of ferrite, the possible frequencies applied to a spark plug are limited to approximately 30 kHz.
U.S. Pat. No. 5,842,456 shows a multi-firing ignition circuit with which several pulses per combustion cycle can be supplied to a spark plug. Concrete circuits are not shown, but the patent does mention that here too use is made of energy from a capacitance in a “capacitive discharge coil-on-plug” circuit. The patent does not speak of the use of an existing breakdown path to generate an oscillating current through the gas mixture with several pulses.
European patent application No. 893600 shows the use of several pulses per combustion. The patent application does not speak of the use of an existing breakdown path to generate an oscillating current through the gas mixture with several pulses.
European patent application 0913897, German patent application DE10037536and PCT patent application 0133073 describes the use of plasma ignitions, in which very high-frequency electromagnetic signal sources, up to inter alia laser and microwave sources, are used to heat gas mixtures. European patent application 0913897 describes a plasma ignition circuit in which the gas mixture is ignited by thermal energy from a high-frequency field. This publication provides for the ignition of different plasma filaments one after the other, by means of a special kind of spark plug. Details about the high-frequency circuit are not given. U.S. Pat. No. 4,366,801 discloses an ignition circuit with a resonant circuit for generating pulses. DE10037536 utilizes signals of about 1 Ghz, but further does not describe any circuits. PCT patent applications 0133073 describes the use of laser light or microwaves of a plasma, after this has been rendered conductive with an electric pulse.
U.S. Pat. No. 4,366,801 discloses the use of a number of transformers, each for its own spark plug, and each in series with a capacitor, with which the transformer forms a resonant circuit, so that a resonant signal is generated across the spark plug. PCT patent application WO0050747 describes an ignition which, after a breakdown generating pulse, generates a sequence of further voltage oscillations across the electrodes of the spark plug, whereby a reactance circuit causes the voltage of the voltage oscillations to decrease once the breakdown path is conductive. If the breakdown path is interrupted, the voltage increases again. This publication shows an oscillator to generate the pulses with a frequency in the band of 1-100 kHz. The pulse forming circuit comprises the transformer and a capacitor which jointly work as a resonant circuit.
As far as these ignition devices work with spark plugs that have electrodes between which electric fields are generated, all these devices utilize a transformer of which the secondary winding is coupled to the electrodes and of which the primary winding is included in a resonant circuit. This enables efficient transfer of the needed energy to the electrodes. However, it also has as an effect that the maximum rise speed of the field across the electrodes is related to the pulse repeat frequency. When effecting the initial breakdown between the electrodes, the circuit must deliver a voltage so high as to be sufficient to create a breakdown path.
U.S. Pat. No. 4,846,129 describes an ignition system wherein a detector is employed to sense the first or “breakdown” phase of spark discharge across a spark plug which causes a short duration high current flow across the plug gap. The detection of the breakdown current enables control over a number of ignition system functions. A pulse transformer is used which enables extremely short duration energization of the spark plug at controllable voltages. The spark plug may be caused to multiply discharge within a short duration which has been found to increase the lean burn limit of the engine. Rapid multiple firing of the plug is achieved by sensing the existence of breakdown current which signifies the discharge event. This signal is used to immediately curtail that discharge cycle and begin another firing cycle, enabling multiple discharges to occur in a very short time duration. However, at each discharge the electrodes of the spark plug are eroded by the repeatedly particle impacts resulting in a relatively short spark plug lifetime.
It is one object of the invention to provide an improved internal combustion engine and an ignition circuit for such an internal combustion engine.
Therefore, the invention relates to an internal combustion engine comprising a cylinder with a spark plug, an electric DC voltage supply and an ignition circuit, which ignition circuit is provided with a switching device and a transformer having a primary winding, which is coupled to the supply via the switching device, and a secondary winding which is coupled to the spark plug, the switching device being arranged for (i.e., configured such that), after having generated an initial breakdown of a breakdown path through a gas mixture between electrodes of the spark plug, switching repeatedly per combustion to produce pulses at the primary winding, with a repeat frequency which is at least so high that the breakdown path remains conductive between consecutive switches per combustion, so that the switching on and switching off of a high current leads to a heating of the breakdown path in order to ignite the gas mixture, wherein the transformer comprises an air core around which the primary and secondary winding are arranged concentrically.
This arrangement is preferable when transforming high frequency pulses. No energy is stored in the core, resulting in a transformer wherein the rise time of the pulses in the secondary winding can be as high as 2 kV/ns or even higher. This very high rise speed leads to a greater energy content in the gas mixture by means of dielectric heating. As a result, a less high voltage can suffice for generating the initial breakdown.
The primary winding is provided with energy from the DC voltage supply repeatedly during the combustion, without making use of energy which is stored in a resonant circuit. By doing this with a sufficiently high frequency, sufficient energy for the combustion can be supplied without energy from a resonant circuit being necessary. Thus, it can suffice to use a primary winding with a low self-induction, which can be charged again faster, so that fewer problems arise at high speeds. A resonant circuit is now not necessary anymore to generate sufficient energy for the combustion.
Further, also, the erosion of the electrode of the spark plug is less because the kinetic energy of the impacting particles is not high enough to disrupt the structural integrity of the electrode material. As the rest of the pulses of the signal lead to an alternating current through the existing breakdown path, without a new breakdown being necessary in the respective combustion cycle, there does not further arise any erosion either. The minimum repeat frequency needed for this purpose depends on the engine design, typical values being, for instance, between 100 kHz and 10 MHz. In the combustion engine according to the invention, the gas mixture is heated by the high frequency current pulses to a temperature at which combustion occurs.
Preferably, in substantially every period of the repeated switching, the same fraction of the energy for combustion of the gas mixture is supplied.
Thus, the energy is supplied, for instance, equally distributed over fifty or more pulses per combustion, or even over eighty or more pulses and preferably a hundred or more pulses per combustion. As a result, no great current per pulse is needed.
The duty cycle and/or the frequency with which the switching device connects the supply with the primary winding may be modulatable. Thus, the amount of energy that is supplied to the spark plug can be set, for instance depending on the condition of the engine, a measured degree of combustion of previous gas mixtures, the available supply voltage, and so forth. Also, the amount of energy of consecutive periods in which the connection is made conductive can be made variable in a single combustion, for instance by supplying more energy per time unit with later pulses to thereby reduce spark erosion.
The frequency with which current pulses from the supply are led through the primary winding is preferably high above the speed of the engine, preferably as high as is practically possible, for instance in a range of 100 kHz-10 MHz. This is simple to realize with a high frequency generator. Thus, a small transformer can suffice, and high speeds of the engine do not entail any decrease of the energy that is supplied to the spark plug.
In an embodiment, the ignition circuit is arranged to produce a high frequency pulse train at the secondary winding wherein a first pulse of the pulse train has an amplitude of at least 2 Kilovolt and subsequent pulses of the pulse train have amplitudes of less than 300 Volt. After having generated an initial breakdown path in the gas mixture, the voltage of subsequent pulses can be much lower than the initial pulse. The number of subsequent lower pulses per combustion may vary, so that the amount of energy transferred to the gas mixture can be regulated.